I personally think 720p is better, however you could argue this both ways.
720p is a progressive signal where all 720 lines are displayed on the screen at the same time, a 1080i signal has 1080 lines however they are not displayed on-screen at the same time.
Without referring to any sources off the top of my head I believe 720p will display 60fps where as 1080i can only display 30fps, the higher the FPS the smoother game play experience you will have.
720p is better suited for fast moving images such as games and sports channels as you don’t get any blur or “stepping effect” as people like to call it, where as you do with 1080i images.